The present invention relates in general to a bulk material container or tank having a manhole cover, and more particularly to a bulk material container or tank having a manhole cover with a pressure-vacuum relief valve.
It is desirable to provide manhole covers for bulk material containers with a pressure-vacuum relief valve. When the fluid pressure within the container or the tank is excessive, the pressure valve of the relief valve of the manhole cover opens to vent the fluid pressure of the fluid within the container or tank to atmosphere. When the fluid pressure of the fluid within the container or the tank is less than atmospheric pressure, the vacuum relief passage of the relief valve of the manhole cover provides a path for the ambient air to enter the container or tank.
The application of a restraining force to secure the relief valve to the manhole cover may distort the manhole cover. The distortion of the manhole cover may influence the effectiveness of the relief valve seal or the seating of the valve plate on the valve seat of the relief valve. Thus, distortion of the manhole cover has an effect on the relieving pressure and the re-sealing pressure of the relief valve.
In the employment of a plate valve for a manhole cover of a bulk material container to provide the initial pressure and vacuum relief, there is limited clearance in the opening of the plate valve because the spring constant inhibits the opening of the plate valve. Thus, the volume of the fluid passing through the opening of the plate volume may be insufficient.
In the U.S. Patent to Drane, U.S. Pat. No. 2,169,410, issued on Aug. 15, 1939, for Breather Valve And Hatch For Tanks, there is disclosed a popoff valve plate. When the fluid pressure in the tank is sufficient to lift the valve plate against the urgency of springs, gas escapes from the tank and is vented to atmosphere. When the fluid pressure in the tank is reduced below atmospheric pressure, an inlet valve is unseated to permit air from the atmosphere to enter the tank to lessen the degree of vacuum in the tank.
The U.S. patent to Tokheim, U.S. Pat. No. 2,152,422, issued on Mar. 28, 1939, for Safety Device For Volatile Liquid Storage Tanks discloses a relief valve for a liquid storage tank. The relief valve is surrounded by an annular series of vacuum valves.
In the U.S. patent to Eshbaugh, et al., U.S. Pat. No. 2,164,450, issued on July 4, 1939, for Radiator Pressure Cap, there is disclosed a vacuum cup valve, which unseats one spring in response to a vacuum. The surrounding enclosure of the one spring will elevate from overpressure against another spring.
The U.S. patent to Konchan, U.S. Pat. No. 2,655,284, issued on Oct. 13, 1953, for Radiator Pressure Cap discloses a vacuum relief valve unseating against the urgency of one spring and a pressure relief valve unseating against the urgency of another spring. The U.S. patent to Uptegraff, Jr., et al., issued on Jan. 25, 1977, for Casing Construction For Pole Type Dielectric Containing Transformer discloses an operating stem carried for rotative movement by a bearing gasket secured to the underside of a cover. A spring is carried on the stem.
The U.S. patent to Kushman, et al., U.S. Pat. No. 4,109,819, issued on Aug. 29, 1978, for Explosion Vent And Method Of Venting discloses a quick-release vent for releasing a rapid pressure buildup within a storage bin. The U.S. patent to Kellogg, U.S. Pat. No. 4,339,054, issued on July 13, 1982, for Pressure Relieved Plug And Socket Cleanout Assembly discloses a pressure relief valve mounted in a plug and is manually operated to vent excess pressure from a receptacle.